1. Field of the Invention
The present invention relates to electrolytes useful in the electrochemical machining of nickel-base superalloys, especially those containing hafnium.
2. Description of the Prior Art
Electrochemical machining is widely used in the gas turbine industry to produce parts, such as blades, vanes and the like, having aerodynamically contoured surfaces and internal cooling passages from materials, such as nickel base superalloys, which are difficult to machine by conventional techniques. Electrochemical machining involves the controlled removal of metal from a part by anodic dissolution in an electrolytic cell in which the part is the anode and the tool is the cathode. Electrolyte is pumped between the tool and part while a direct current is passed through the cell at a low voltage so as to dissolve metal from the part.
Electrolytes commonly used in the prior art to machine nickel base superalloys involve aqueous solutions of either inorganic salts or strong mineral acids. Typical of the former is an aqueous solution containing 1 pound per gallon of NaCl and 1 to 3 ounces per gallon of NaNO.sub. 3. Aqueous solutions containing NaCl and NaNO.sub. 3 are also used to electrochemically machine other metals and alloys, as illustrated in U.S. Pat. Nos. 3,284,327 and 3,401,103 and Russian Pat. Nos. 281,984 and 324,299.
Unfortunately, the prior art aqueous electrolyte solutions have not always produced satisfactory results in terms of providing a desirable surface finish on the machined nickel base superalloy part. Oftentimes, the machined surface exhibits pits, microcracks and other defects as a result of preferential anodic dissolution of certain phases or morphological features, such as grain boundaries, of the nickel base superalloy. Such surface defects are highly deleterious since they can cause premature failure of the part during service in the harsh environment of the gas turbine engine. Especially unsatisfactory results have been observed with nickel base superalloys containing hafnium, such as that known generally as MAR-M200+Hf comprising (nominally) 9% Cr, 10% Co, 2% Ti, 5% Al, 0.15% C, 12% W, 1% Cb, 2% Hf, 0.015% B, balance essentially nickel. Although the hafnium-bearing nickel base superalloys exhibit useful properties, their application in gas turbine engines has been somewhat limited due to the inability of the prior art to electrochemically machine the material with the desired defect-free surface.
One attempt to provide a smooth, defect free surface on nickel base superalloys, including those containing hafnium, is illustrated in copending U.S. application Ser. No. 638,209 entitled "Electrolyte for Electrochemical Machining of Nickel Base Superalloys" and assigned to the assignee of the present invention. The electrolyte disclosed therein comprises an aqueous solution of strong mineral acids, including nitric and hydrochloric acids, and citric acid.